An active matrix type liquid crystal display using switching elements such as thin film transistors (TFTS) begins to spread widely as a display terminal of an office automation apparatus, etc. The display type of such a liquid crystal display is roughly divided into the following two kinds. One is a method in which the liquid crystal is sandwiched by two substrates on which transparent electrodes are formed, and it is operated by applying a voltage to the transparent electrodes, and the display is performed by modulating the light penetrated the transparent electrodes and launched into the liquid crystal. The other is a method in which the liquid crystal is sandwiched by two substrates on which transparent electrodes are formed, and it is operated by applying an electric field substantially parallel to one substrate and between two electrodes (a pixel electrode and an opposed electrode) formed on the substrate, and the display is performed by modulating the light incident from the space of one electrode into the liquid crystal. Because this method has features such as a wide range of viewing angles and low load capacitance, etc, it is a promising technology concerning the active matrix type liquid crystal display. Hereinafter, the latter method is called a horizontal electric field method.
While the horizontal electric field method has the above-mentioned features, there is a problem that it is necessary to use a bright back light of which electric power consumption is large, because opaque electrodes are formed like comb teeth. It is because the frontage area through which light can be penetrated is small and thus display screen is dark.
On the other hand, the same applicant has proposed one method in the Japanese Patent Application No. 6-199247, in which the role of the common electrode wiring of supplying an external voltage to the common electrode is given to the scanning electrode wiring, the common electrode wiring is eliminated, and thus the frontage area in the horizontal electric field method is enlarged. Hereinafter, the above-mentioned method technology will be called horizontal electric field method having no common-electrode-wiring (common-less horizontal electric field method).
In the common-less horizontal electric field method, it is required that the thin film transistor or the switching element indicates fully the enhancement type switching characteristic that the threshold voltage is higher than the maximum voltage of the liquid crystal operating voltage required to modulate optically the liquid crystal. As a method of making the thin film transistor which shows the enhancement type switching characteristic, a method of making a thin film of the amorphous silicon semiconductor layer or a method of tLr controlling the voltage of the back electrode provided at a position opposed to the gate scanning electrode of the thin film transistor has been described in the above Japanese Patent Application No. 6-199247.
However, in these methods, it is necessary to reduce enough the difference of threshold voltages of the THIN FILM TRANSISTOR. There is a problem that the display quality of the active matrix type liquid crystal display having no common-electrode-wiring is low.
An object of the present invention is to provide an active matrix type liquid crystal display having no opposed-electrode-wiring of the horizontal electric field method, in which the difference of threshold voltages are reduced, thin film transistors with simple structure are used, and the picture resolution is improved.